1. Field of the Invention
The present invention relates to a method of establishing a color conversion relationship for converting color signals from a first colorimetric system to a second calorimetric system.
2. Description of the Related Art
For recording a full-color image with an output device such as a color printer, a printing press, or the like, it is possible to process color signals Y, M, C, K (yellow, magenta, cyan, black) to produce a color image having desired colors. Since the colors of the color images depend on a color reproduction process, e.g., a process of generating a gradation image based on halftone dot modulation or a process of generating a gradation image based on density modulation, of the output device and the characteristics of an output medium used by the output device, it is necessary to generate color signals Y, M, C, K through a color conversion effected in view of color reproduction characteristics based on the above process and output medium characteristics.
Specifically, color signals R, G, B (red, green, blue) which have been inputted from an input device are converted into signals of a common color space which comprise stimulus signals X, Y, Z of a CIE standard calorimetric system, and the signals are then outputted from an output device whose color reproduction characteristics are defined by the common color space, thereby producing a color image on an output medium independently of the input and output devices and the output medium.
More specifically, a plurality of color patches having different colors are generated using the output device and the output medium and then calorimetrically measured thereby to obtain a conversion relationship (hereinafter referred to as a "forward conversion relationship") for converting color signals Y, M, C, K of the color patches to stimulus signals X, Y, Z of a common color space which do not depend on the output device and the output medium. Then, a conversion relationship (hereinafter referred to as a "reverse conversion relationship") for converting the stimulus signals X, Y, Z to color signals Y, M, C, K is obtained from the forward conversion relationship. Thereafter, the stimulus signals X, Y, Z are converted to color signals Y, M, C, K using the reverse conversion relationship. In this manner, the color signals Y, M, C, K thus produced have the color reproduction characteristics of the output device and the output medium.
FIGS. 8A and 8B of the accompanying drawings show two-dimensional representations of a forward conversion relationship (FIG. 8A.fwdarw.FIG. 8B) and a reverse conversion relationship (FIG. 8B.fwdarw.FIG. 8A) between known color signals Y, M, C, K from color patches generated at regular grid intervals and stimulus signals X, Y, Z. According to these forward and reverse conversion relationships, the color signals Y, M, C, K at regular grid points and the stimulus signals X, Y, Z at points of intersection of irregular curves correspond to each other. The intervals between the points of intersection are highly irregular in the color space of the stimulus signals X, Y, Z. Therefore, for determining color signals Y, M, C, K corresponding to stimulus signals X, Y, Z at a point A, color signals Y, M, C, K at points Al, A2, A3, A4 should be interpolated, but in fact color signals Y, M, C, K at points A2, A3, A4, A5 tend to be interpolated as these point are also close to the point A. To avoid the above drawback, it is necessary to determine a relationship between color signals Y, M, C, K and stimulus signals X, Y, Z using as many color patches as possible. However, if color patches are to be generated with halftone dot percentages of 10% intervals, then a total of 10.sup.4 color patches will be required. An increase in the number of color patches generated necessarily results in an increase in the time needed to calorimetrically measure the color patches. The color reproduction characteristics include not only the characteristics of the output device, but also the characteristics of the output medium. Specifically, because the characteristics of the output medium depend on many factors including the types of output sheets and the types of color materials used, for example, the number of color patches which are required is further increased if all those factors are to be taken into account. If the reverse conversion relationship thus generated is to be kept as a print color conversion table as disclosed in Japanese laid-open patent publication No. 3-131920, for example, then a very large number of data will be required, making such a print color conversion table impractical.